1. Field of the Invention
This invention relates to a thermal recording element for recording information by thermally deforming a recording layer using a high intensity radiation and recording information by the change in the light transmitting and reflecting properties of the layer. More particularly, the invention relates to a thermal recording element having protective layers having high mechanical strength and exhibiting reduced reduction in recording sensitivity.
2. Description of the Prior Art
When a recording layer having high optical density is irradiated by a high intensity radiation such as a laser beam in accordance with information, the recording layer by absorbing the high energy density light beam thus irradiated undergoes local temperature increases producing physical changes such as fusion, evaporation, aggregation, etc., whereby information is recorded by the formation of a difference in optical density between the irradiated portions and non-irradiated portions. Such a thermal recording element has the advantages that processing such as development, fixing, etc., are unnecessary, a dark room is unnecessary since the recording element is not sensitive to ordinary room light, high contrast images are obtained, and addition recording (add on) of information is possible.
Recording on such a thermal recording element is generally performed by converting recording information to an electric time-succession signal and scanning the recording element by a laser beam the intensity of which is modulated according to the electric signal. In this case, there is also the advantage that the recorded image is obtained in real time.
Such recording elements are described in, for example, M. L. Levene, et al, Electron, Ion and Laser Beam Technology (the record of the 11th Symposium held in 1969; "Elctronics", page 50 (March 18, 1968); D. Maydan, The Bell System Technical Journal, Vol. 50, 1761 (1971); C. O. Carlson, Science, Vol. 154, 1550 (1966), etc. As the recording layer for the thermal recording elements, metals, dyes, plastics, etc., are suitable. In particular, as a metal recording layer, there is a thermal recording element comprising a support having formed thereon a thin layer of a metal such as Bi, Sn, In, etc., and such a recording element has excellent properties as a thermal recording element such that images of high resolving power and high contrast can be recorded.
However, the recording element containing a thin metal recording layer generally shows light reflectance of higher than 50% to the laser light used for recording, which makes it impossible to utilize effectively the energy of the laser beam and hence power of the laser light used for recording must be higher which requires a laser light source of high output for recording at high scanning speed, and thus the recording apparatus used becomes larger and more expensive.
Therefore, various recording elements having high recording sensitivity have been investigated and as an example, a recording element wherein a very thin layer of Ge is superposed on a thin layer of Se and Bi for reducing the light reflectance is described in U.S. Pat. No. 3,560,994. However, the use of Se, etc., is undesirable since there is a possiblity of toxicity problems and the images recorded are not satisfactory.
As other examples of a recording element having a reflection preventing layer, a recording element having a metal layer and a reflection preventing layer absorbing light in the wave length region of the laser light used for recording is disclosed in the specifications of Japanese Patent Application (OPI) Nos. 151,151/'75 and 74,632/'76. Also, a recording element having a recording layer composed of a physical mixture of a metal and a reflection preventing material is disclosed in Japanese Patent Application (OPI) No. 78,236/'76. However, the recording layer of such a thermal recording element is generally very liable to be scratched and since the thickness of the recording layer is at most about 0.5 .mu.m, the scratches formed on the surface of the recording layer during handling the recording element reach the surface of the support and they can appear upon reading the recorded information, which results in a loss in the reliability of the recorded information. Furthermore, in such thermal recording elements it is very difficult to select a thermal recording element having sufficient sensitivity, high mechanical strenght, and sufficient adaptability for practical use and further it is difficult to provide such recording elements commercially at a low cost.
Hitherto, to overcome the scratching difficulty, it is know to form a protective layer on the recording layer and recording elements having a protective layer of an organic high molecular weight material or polymer on the recording layer are disclosed in Japanese Patent Application (OPI) Nos. 96,716/'74, 59,626/'76, 75,523/'76 and 88,024/'76. However, when the thickness of the protective layer composed of such an organic high molecular weight material or polymer is sufficiently increased to prevent the recording layer from being scratched, for example, made thickener than 3 .mu.m, the sensitivity of the recording element is reduced greatly, (e.g., to 1/2 or 1/3 of the original sensitivity) which makes the recording element impractical.